The invention relates to a process for preparing a transfer metallization film which is metallized on one side, and to a transfer metallization film based on a biaxial stretch-oriented multilayer film prepared by coextruding polypropylene.
For specific packaging purposes there exists a high demand for composite materials composed of paper and metal, in particular aluminum foil, which increases from year to year. Although extremely thin metal foils are already used, and are indeed adequate, for such composite materials, further savings of the scarce and costly metal by the use of metallized papers are desirable. The metallization of paper is carried out in the state of the art by two different processes. In the first process, paper is directly coated with metal vapor in a high vacuum, while in the second process an extraneous substrate is coated with a metal layer, and the latter is then transferred to paper. The second process is also referred to as transfer metallization.
The direct vapor deposition onto paper is crucially affected by the choice of a suitable grade of paper, and it further requires a costly pretreatment of the paper. This pretreatment comprises either sealing the paper with a coat of lacquer, or a drying step whereby the normal water content of the paper is reduced from 6-7% down to about 3%. After the deposition of metal vapor, the paper must then be brought back to the normal water content. These steps are costly and time consuming.
In the process of transfer metallization, a plastic film, usually based on polyolefin or polyester, is subjected to vapor deposition in a high vacuum. The metallized film is coated with a contact adhesive on the metallized side and is brought into contact with the paper to be metallized, which can in this case be chosen as desired, according to the requirements of the end use. After the full curing of the contact adhesive, the plastic film is peeled off the composite, leaving the metal layer behind on the paper. The process of transfer metallization can be carried out economically only if the plastic film can be used repeatedly. Conventional films can normally be reused up to six times.
German Patent Application P 34 34 298.2 describes a film made of thermoplastic polymer, which, in the transfer metallization from the film onto paper, is economically reusable up to 20 times. However, even the first metallization step gives rise to a serious disadvantage which considerably impairs product quality and hence the utility of the film and of the process. This disadvantage takes the form of flash discharges which, for example, during the vapor deposition of the metal layer onto the film, lead to so-called "Lichtenberg figures". These figures impair the uniform appearance of the metal layer and, after the transfer of the metal layer to the paper or plastic film base material, cause the appearance of the base material to become unsightly and thus unusable for predominantly decorative purposes.
Various attempts have been made to overcome this problem. Such attempts include metallizing the second side of the film for the purpose of conducting away the charge, intensive discharging of the film before metallization by means of electric discharging means, antistatic finishing of the film's raw material or of the film surface, and flame treatment of the nonmetallized side of the film. However, not only have all of these techniques failed to produce the desired success, but they make the manufacturing process for the transfer metallization film more complicated and hence, in the final analysis, more expensive.